1. Field of the Invention
The present invention relates generally to a semiconductor device and, more particularly, to a ferroelectric memory device as well as a method of fabrication a ferroelectric memory device.
2. Description of the Related Art
A requirement of modern data processing systems is that a substantial portion of the information stored in memory be randomly accessible to ensure rapid access to such information. Due to the high speed operation of memories implemented in semiconductor technologies, ferroelectric random access memories (FRAMs) have been developed which exhibit a significant advantage of being non-volatile. This non-volatility of FRAMs is achieved by virtue of the fact that a ferroelectric capacitor includes a pair of capacitor plates with a ferroelectric material between them having two different stable polarization states, which can be defined with a hysteresis loop depicted by plotting the polarization against applied voltage.
Recently, the use of such ferroelectric materials has reached commercial applications in the semiconductor industry. Ferroelectric memory elements are non-volatile, are programmable with a low voltage, e.g., less than 5 V, whereas typical flash memories are programmed at 18-22 V, have fast access times on the order of less than a nano-second, whereas typical flash memories have access times on the order of a micro-second, and are robust with respect to virtually unlimited numbers of read and write cycles. These memory elements also consume low power, less than 1 micro-ampere of standby current, and exhibit radiation hardness.
Ferroelectric materials which have allowed this breakthrough in integrated circuit applications include perovskite structure ferroelectric dielectric compounds, such as lead zirconate titanate PbZr.sub.x Ti.sub.1-x O.sub.3 (PZT), barium strontium titanate (BST), PLZT (lead lanthanum zirconate titanate), and SBT (strontium bismuth tantalum).
In a ferroelectric memory fabrication process, it is a key point to obtain ferroelectric characteristics without any degradations, as well as a one capacitor/one transistor structure and a multi-level metal structure. Particularly in the case of a PZT, the ferroelectric characteristics are directly related to the amounts of perovskite crystalline structure produced by post-deposition annealing. Since a PZT film is formed in a heterogeneous manner, formation of the perovskite crystalline structure by post-deposition annealing is greatly affected by the material in contact therewith, such as capacitor electrodes (i.e., the lower electrode and the upper electrode). In particular, a platinum catalyses reduction reaction easily oxidizes the PZT, thereby causing unacceptable defects in the interface between the electrodes and the PZT as well as causing a deficit in the amount of titanium, which is easily oxidized, in the PZT, and which eventually results in reliability concerns.